1. Field of the Invention
The invention relates to a circuit arrangement for regenerating the black level of video signals during their A/D conversion, in which a deviation of the black level of the digital video signals from a predetermined value influences the black level of the analog video signals.
2. Description of the Related Art
Video signals generally comprise a black reference level, hereinafter also referred to as black level, per line within the blanking interval. The black level is already generated in image pick-up devices, such as, for example, CCD frame sensors, and mainly serves as a reference value after transmission of the video signals. To recover the DC value lost during transmission, known clamping and black level control circuits are used.
If an analog video signal is to be subjected to an A/D conversion, referencing to the black level is also necessary in order that a predetermined value in the generated digital video signals corresponds to the black level as accurately as possible. For this purpose, circuits are known from, for example, U.S. Pat. No. 5,084,700 or U.S. Pat. No. 5,087,973, which, in the digital plane, perform a nominal-real value comparison for the position of the black level and derive analog adjusting values therefrom. The black level of the analog video signal is shifted by adding the correction signal. The correction value computed in the digital plane always influences the entire subsequent line. It is, therefore, possible that the black level jumps in amplitude from line to line. This may lead to unwanted line-frequent noise effects.
It is therefore an object of the present invention to provide a circuit arrangement for regenerating the black level in which these unwanted noise effects do not occur.
In the circuit arrangement according to the invention, this object is achieved in that the scanning values obtained during A/D conversion of the black level of the analog video signals are compared with the predetermined value, and in that a ramp (or sawtooth) signal for tendency-compensating the deviations is superimposed on the analog video signals. Particularly, the comparison is performed during the blanking intervals, and the ramp signal extends through the subsequent line.
To prevent disturbances due to addition of the ramp signal, also in critical image contents, for example, in areas of a single color, the circuit arrangement according to the invention may be characterized in that the amplitude of the ramp signal is smaller than a quantization step in the A/D conversion.
A further embodiment of the circuit arrangement according to the invention is characterized in that the differential values, obtained in the comparison of the scanning values with the predetermined value, are added with the correct sign during a blanking interval, and in that the result of the addition is used to generate the ramp signal in such a way that the ramp signal is zero at a result within the predetermined limits, positive at a result below the predetermined limits, and negative at a result above the predetermined limits.
Due to these measures, a suppression of the noise superimposed on the black level is simply possible when using the ramp signal. This further embodiment may be realized without extensive computations when the differential values are limited to the states  greater than ,  less than  and = and when the addition is effected by way of up/down counting.
An advantageous embodiment of the invention is characterized in that a capacitor and a current source are provided for generating the ramp signal, this current source being controllable for supplying a positive current, a negative current, or no current.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.